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multi dimension array initialization using pointers

 
 
natkw1@yahoo.com.au
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Posts: n/a
 
      10-08-2005
Hi,

I'm attempting to understand the use of pointers(at least grasp how
pointers work). I've read the FAQ on
http://www.eskimo.com/~scs/C-faq/s6.html on pointers and arrays but I'm
still a bit lost.

I written the following code to try to understand it but it's not
working:

#include <stdio.h>
#include <ctype.h>

#define MAXROW 2
#define MAXCOL 5

void init_array(char *data[MAXROW]);
void print_array(char *data[MAXROW]);


int main ()
{
char *array_ptr[MAXROW];

init_array(array_ptr);
print_array(array_ptr);
return 0;
}

void init_array(char *data[MAXROW])
{
int row, col;

for (row=0; row<MAXROW; row++)
{
for (col=0; col<MAXCOL; col++)
{
(data[row] + col) = '\0';
}
}
}

void print_array(char *data[MAXROW])
{
int row, col;

for (row=0; row<MAXROW; row++)
{
for (col=0; col<MAXCOL; col++)
{
printf("%c",*(data[row] + col));
}
}
printf("\n");
}

When I try to compile it with gcc, there is a warning
a3.c: In function `init_array':
a3.c:29: invalid lvalue in assignment

Can anyone tell me what the warning means ?
What I am doing wrong ?

Nat

 
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pete
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Posts: n/a
 
      10-08-2005
http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
>
> Hi,
>
> I'm attempting to understand the use of pointers(at least grasp how
> pointers work). I've read the FAQ on
> http://www.eskimo.com/~scs/C-faq/s6.html on pointers and arrays but I'm
> still a bit lost.
>
> I written the following code to try to understand it but it's not
> working:
>
> #include <stdio.h>
> #include <ctype.h>
>
> #define MAXROW 2
> #define MAXCOL 5
>
> void init_array(char *data[MAXROW]);
> void print_array(char *data[MAXROW]);
>
> int main ()
> {
> char *array_ptr[MAXROW];
>
> init_array(array_ptr);
> print_array(array_ptr);
> return 0;
> }
>
> void init_array(char *data[MAXROW])
> {
> int row, col;
>
> for (row=0; row<MAXROW; row++)
> {
> for (col=0; col<MAXCOL; col++)
> {
> (data[row] + col) = '\0';
> }
> }
> }
>
> void print_array(char *data[MAXROW])
> {
> int row, col;
>
> for (row=0; row<MAXROW; row++)
> {
> for (col=0; col<MAXCOL; col++)
> {
> printf("%c",*(data[row] + col));
> }
> }
> printf("\n");
> }
>
> When I try to compile it with gcc, there is a warning
> a3.c: In function `init_array':
> a3.c:29: invalid lvalue in assignment
>
> Can anyone tell me what the warning means ?
> What I am doing wrong ?


/* BEGIN new.c */

#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>

#define MAXROW 2
#define MAXCOL 5

void init_array(char **data);
void print_array(char **data);
void free_array(char **data);

int main (void)
{
char *array_ptr[MAXROW];

init_array(array_ptr);
print_array(array_ptr);
free_array(array_ptr);
return 0;
}

void init_array(char **data)
{
int row;

for (row = 0; row < MAXROW; row++) {
data[row] = malloc(MAXCOL);
if (data[row] == NULL) {
puts("malloc");
exit(EXIT_FAILURE);
}
assert(MAXCOL >= sizeof "data");
strcpy(data[row], "data");
}
}

void print_array(char **data)
{
int row;

for (row = 0; row < MAXROW; row++) {
fputs(data[row], stdout);
}
putchar('\n');
}

void free_array(char **data)
{
int row;

for (row = 0; row < MAXROW; row++) {
free(data[row]);
}
}

/* END new.c */


--
pete
 
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Moritz Beller
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Posts: n/a
 
      10-08-2005
On 8 Oct 2005 00:02:35 -0700
(E-Mail Removed) wrote:

> #define MAXROW 2
> #define MAXCOL 5


Why not const int MAXROW = 2; ?

> void init_array(char *data[MAXROW])
> {
> int row, col;
>
> for (row=0; row<MAXROW; row++)
> {
> for (col=0; col<MAXCOL; col++)
> {
> (data[row] + col) = '\0';


This won't work. Should be:
data[row + col] = '\0';

> }
> }
> }


data's got 5 (0..4) char* elements. You are trying to access elements
0..9 in this for loop, which of course won't work out well.

Same in here:

> void print_array(char *data[MAXROW])
> {
> int row, col;
>
> for (row=0; row<MAXROW; row++)
> {
> for (col=0; col<MAXCOL; col++)
> {
> printf("%c",*(data[row] + col));


This won't work. Should be:
printf("%c",data[row + col]);

> }
> }
> printf("\n");
> }


Either change your array declaration to char *array_ptr[MAXROW*MAXCOL];
or leave the second for-loops out.

BTW: void print_array(char *data[MAXROW]) is really bad style. You
should rather supply the number of elements in the array as the second
argument: void print_array(char **data, int elements)
Call with print_array(data, MAXCOL*MAXROW);

best regards / Gruß
Moritz Beller
--
web http://www.4momo.de
mail momo dot beller at t-online dot de
gpg http://gpg.notlong.com
 
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Michael Mair
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Posts: n/a
 
      10-08-2005
Moritz Beller wrote:
> On 8 Oct 2005 00:02:35 -0700
> (E-Mail Removed) wrote:
>
>>#define MAXROW 2
>>#define MAXCOL 5

>
> Why not const int MAXROW = 2; ?


Because in C, const does not generate compile time constants.
So, for pre-C99 C, you cannot use it for array sizes at
declaration. Or case labels.
You are talking about another language, maybe C++.

Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.
 
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natkw1@yahoo.com.au
Guest
Posts: n/a
 
      10-08-2005
I tried the suggestion of using data[row + col] in both functions but
when compiling, I now get an message in the init_array function,

warning: assignment makes pointer from integer without a cast

void init_array(char *data[MAXROW])
{
int row, col;

for (row=0; row<MAXROW; row++)
{
for (col=0; col<MAXCOL; col++)
{
data[row + col] = '\0';
}
}
}

I had also tested changing my array declaration to
char *array_ptr[MAXROW*MAXCOL]but the same warning comes up.

But when I use data[row][col] instead of (data[row] + col) or data[row
+col] it works.
I'm more confused.
So when does the pointers "bits and pieces" get used in the function ??


Nat

 
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Michael Mair
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Posts: n/a
 
      10-08-2005
Please quote a sufficient amount of context -- it is not guaranteed
that people see any other message from this thread before this one.
Also, this was Moritz Beller's suggestion, not mine.

(E-Mail Removed) wrote:
> I tried the suggestion of using data[row + col] in both functions but
> when compiling, I now get an message in the init_array function,
>
> warning: assignment makes pointer from integer without a cast
>
> void init_array(char *data[MAXROW])
> {
> int row, col;
>
> for (row=0; row<MAXROW; row++)
> {
> for (col=0; col<MAXCOL; col++)
> {
> data[row + col] = '\0';


You have to pass an array of char, not an array of pointers to
char. data is of the wrong type for that. If you have a parameter
char *data (instead of char *data[MAXROW])for the function and
data points to a storage area of at least MAXROW*MAXCOL bytes, then
data[row*MAXCOL + col] = '\0';
may be what you are looking for.

> }
> }
> }
>
> I had also tested changing my array declaration to
> char *array_ptr[MAXROW*MAXCOL]but the same warning comes up.


See above.

> But when I use data[row][col] instead of (data[row] + col) or data[row
> +col] it works.
> I'm more confused.
> So when does the pointers "bits and pieces" get used in the function ??


a[i] is effectively a+i, so the former two are the same. The latter,
though, cannot be true.
However, without context I am too lazy trying to guess what you
originally wanted to achieve and thus cannot help you with your
problem.


-Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.
 
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Moritz Beller
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Posts: n/a
 
      10-08-2005
On 8 Oct 2005 04:43:42 -0700
(E-Mail Removed) wrote:

> But when I use data[row][col]


This could compile, but will usually seg fault! You cannot assign
anything but an array of chars to data[row]! data[row][col] = '\0';
will not work at all, as long as you don't have an array assigend to
data[row], whose size is at least col+1!

I give you an exmaple to understand things easier: You declare data to
be a one-dimensional array of type char*! It's not a multidimensional
array after all! So data[0] is effectivley pointing to a pointer to
char.

You virtually assgin data[0][n] in the for loop, thereby dereferencing
this very pointer. You cannot assign data[0][n] with a character,
though, as it's at this moment just a pointer of char that you have
declared. This pointer should usually point to another array of char!
Then (as mentioned above) data[0][n] is entriley valid.

(data[row] + col) or data[row+col] are all just de-referencing once.
When being printed, they should return the whole string (*(test + row *
col) is equal to data[row]).

If that's what you are trying to achieve, then declare char
*array_ptr[MAXROW] to be char *array_ptr[MAXROW][MAXCOL] and your
program will perhaps work.

data[row + col] = '\0'; ->
**(data + row * col) = '\0'; or, yet easier:
data[row][col]

best regards / Gruß
Moritz Beller
--
web http://www.4momo.de
mail momo dot beller at t-online dot de
gpg http://gpg.notlong.com
 
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Barry Schwarz
Guest
Posts: n/a
 
      10-08-2005
On 8 Oct 2005 00:02:35 -0700, (E-Mail Removed) wrote:

>Hi,
>
>I'm attempting to understand the use of pointers(at least grasp how
>pointers work). I've read the FAQ on
>http://www.eskimo.com/~scs/C-faq/s6.html on pointers and arrays but I'm
>still a bit lost.
>
>I written the following code to try to understand it but it's not
>working:
>
>#include <stdio.h>
>#include <ctype.h>
>
>#define MAXROW 2
>#define MAXCOL 5
>
>void init_array(char *data[MAXROW]);
>void print_array(char *data[MAXROW]);
>
>
>int main ()
>{
> char *array_ptr[MAXROW];


array_ptr is an array of MAXROW pointers to char. If we assume for
discussion that a pointer to char is 4 bytes, then you have a block of
memory MAXROW*4 bytes long reserved for this array. However, these
bytes are not initialized. Their value is indeterminate.
Colloquially, none of the pointers actually point anywhere yet because
they have not been initialized.

>
> init_array(array_ptr);


Normally, evaluating an initialized object would cause undefined
behavior. However, an expression of type array has the special
property that in most contexts, including this one, evaluating the
expression produces a value equal to the address of the first element
with type pointer to element type. So the use of array_ptr here is
well defined and exactly equivalent to &array_ptr[0].

> print_array(array_ptr);
> return 0;
>}
>
>void init_array(char *data[MAXROW])
>{
> int row, col;
>
> for (row=0; row<MAXROW; row++)
> {
> for (col=0; col<MAXCOL; col++)
> {
> (data[row] + col) = '\0';


Here is where you get into trouble. Consider the iteration when row
is 0.

data[0] is the first pointer in array_ptr from main. You never
initialized this pointer to point anywhere. Attempting to evaluate an
indeterminate value invokes undefined behavior.

Consider the code segment
int i;
i = i + 1;
Because i is not initialized, there is no previous value to add 1 to
and this invokes the same undefined behavior. However, with either
int i = 5;
i = i + 1;
or
int i;
i = 5;
...
i = i + 1;
there is a previous value of i to add 1 to and both result in i having
the value 6.

The same situation is true for you pointer. In order for this
expression to evaluate properly, you must first assign it a value. In
this case, that value must be the address of a char. Since you intend
to use this pointer to reference a row of the matrix, it needs to
point to the first of a sequence MAXCOL char. There are two common
approaches for this:

One is to allocate space dynamically:
data[0] = malloc(MAXCOL * *data[0]);

The other is to define an array of char and use the address of
the array:
char row0[MAXCOL];
...
data[0] = row0;

Now that data[0] evaluates properly, we address the rest of the
expression. On the first iteration, col is also 0. data[0] has type
pointer to char and 0 has type int so this is an example of pointer
arithmetic. The expression evaluates to the address 0 bytes past the
char that data[0] points to and has type pointer to char.

So your are attempting to assign the character '\0' (which is
really an int) to an l-value that has type pointer. Normally,
attempting to assign an int to a pointer is a syntax error and would
result in a diagnostic about incompatible types. However, any integer
expression that evaluates to 0, as '\0' does, is recognized as a valid
NULL pointer constant which can be assigned to a pointer to char.

So while you don't have that syntax error, this would not do what
you want. The error message you receive is telling you that the
expression on the left of your assignment statement is not acceptable
as the recipient of a value. You want to assign the character '\0' to
the char that your pointer expression points to. To do this, you need
to dereference the pointer because you want to manipulate the object
it points to.

The dereference operator (*) would work
*(data[row] + col) = '\0';
but the normal C idiom for this is
data[row][col] = '\0';
> }
> }
>}
>
>void print_array(char *data[MAXROW])
>{
> int row, col;
>
> for (row=0; row<MAXROW; row++)
> {
> for (col=0; col<MAXCOL; col++)
> {
> printf("%c",*(data[row] + col));


You have used the dereference operator properly here.

If you correct the problems with array_ptr not being initialized
properly, you will still have a problem printing this way. Your
entire matrix is filled with '\0'. This is normally not a printable
or displayable character. Attempting to print it with %c will prove
very unsatisfactory.

You could print it with %d (and get 0) or you could choose to
initialize the matrix with printable characters.

> }
> }
> printf("\n");


You probably want this up one line so it executes at the end of every
row and not just at the end of the matrix.

>}
>
>When I try to compile it with gcc, there is a warning
>a3.c: In function `init_array':
>a3.c:29: invalid lvalue in assignment


(data[row]+col) does not define an object that can receive a value.

>
>Can anyone tell me what the warning means ?
>What I am doing wrong ?
>
>Nat



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Keith Thompson
Guest
Posts: n/a
 
      10-08-2005
(E-Mail Removed) writes:
> Hi,
>
> I'm attempting to understand the use of pointers(at least grasp how
> pointers work). I've read the FAQ on
> http://www.eskimo.com/~scs/C-faq/s6.html on pointers and arrays but I'm
> still a bit lost.
>
> I written the following code to try to understand it but it's not
> working:
>
> #include <stdio.h>
> #include <ctype.h>
>
> #define MAXROW 2
> #define MAXCOL 5
>
> void init_array(char *data[MAXROW]);
> void print_array(char *data[MAXROW]);

[snip]

These function declaration are likely to cause confusion. They appear
to declare data as an array parameter, of type "array 10 of pointer to
char", but in fact C doesn't have array parameters. Since array names
are implicitly converted to pointers in most contexts, the language
allows you to declare something that *appears* to be an array
parameter, but it's adjusted to be a pointer parameter. The MAXROW
between the brackets is ignored.

It's better to declare your functions as

void init_array(char *data[]);
void print_array(char *data[]);

or even as

void init_array(char **data);
void print_array(char **data);

--
Keith Thompson (The_Other_Keith) (E-Mail Removed) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
 
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natkw1@yahoo.com.au
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Posts: n/a
 
      10-08-2005
Thank you everyone for explaining pointer usage to a C newbie. The in
depth explanation of where my simple code was wrong has been extremely
helpful.

Following the suggestions made, I changed the way I declared my
function, allocated an initial block of memory, used deferencing
operator correctly and everything works.

Nat

 
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